Method of drying gases



Patented July s, 1949 METHOD OF DRYING GASES Walter F. Rollman,Cranfotd, N. J., assignor to Standard Oil Development Company, acorporation of Delaware ApplicationMarch 17, 1944, Serial No. 526,907

3 Claims. (Cl. 62-470) The present invention relates to improvements inthe drying of gases and, more particularly, relates to an improvedprocess in which gases, such as air, carbon dioxide. nitrogen, petroleumand natural gases, and others, are dried by contact with a cooled,granular, inert, solid material which is suspended in thewater-saturated or partially water-saturated gas during the dryingoperation. This application is a continuation-inpart of my abandonedapplication, Serial No.

472,297, filed January 13, 1943, entitled, Chemical process.

Prior to my invention, a commonly used method of drying air to a verylow moisture content, as

for example in blast furnace service, consisted merely of freezing outthe water by passing the air over refrigerated tubes. This method isinefficient because the temperature differential between the refrigerantand the air must be high, due to the low thermal conductivity of thefrost on the tubes; and two units must be used alternately to permitcontinuity of operation since the ice deposited on the tubes must beperiodically removed, and in order to do this the refrigeration plantbeing defrosted must be out of service.

In the dehydration of natural gas, absorption, adsorption orrefrigeration are the methods commonly employed. One of the earliestmethods was that in which brine served as the means for chilling the gasto remove water vapor. Under high pressures of gas, the formation of gashydrates,

crystalline compounds of water and gas, is likely to occur.

Th main object of this invention is to provide a method for dryinggases, particularly air and natural gas, continuously, emciently andrelatively at a much lower cost than heretofore possible.

A specific object of my invention relates to an improvement in thedrying of air to make it suitable, for example, to support thecombustion of a blast furnace, by contacting the moisture-laden air witha cooled circulating granular solid material at temperaturessufficiently low to cause particles of ice to form whereby the moistureis removed from the air, while at the same time the movement of thesolid material prevents the ac cumulation of ice films anywhere in thedryer.

A further specific object is the provision of improved methods fordrying natural gas, to increase pipe line capacity by preventing theformation of clogging ice deposits and to reduce corrosion due to thepresence of excessive amounts of water.

Other and further objects of my invention will appear from the followingmore detailed description and claims.

In the accompanying drawing, I have shown a diagrammatic layout in whichin Fig. 1, I have indicated a complete process for drying air, utilizingthis as typical of other uses ,of my invenm exit pipe 45.

2 tion; and in Fig. 2, I have shown an alternate form of the dryerelement proper.

In order to increase the understanding of my invention, I shall nowdescribe a preferred modification thereof in detail, and in so doing Ishall refer to the drawings in detail. In the views similar referencecharacters refer to similar parts.

In Fig. 1, wet air, say 80% saturated at a temperature of F., isdischarged into the system through line I and thence passes in seriesthrough two heat exchangers 3 and 5 where it is cooled by heatinterchange with ice and cold air from a subsequent portion of thesystem, as will subsequently appear. The chilled air at a temperature ofabout 40 F. is withdrawn through an eductor M which must be at theextreme bottom of the dryer to prevent plugging of the inlet line withfrost. The moist air contacts in a centrally disposed tube I5 an inert,chilled granular solid material such as sand, the said material having aparticle size of, say, from 20-50 mesh. The incoming air is very quicklycooled by contact with the granular solid material in tube l5 and byindirect heat transfer with a coolant which is discharged into coolingspace 20 through inlet line 25 and withdrawn through line 30. Thecoolant material may be brine and may have an inlet temperature of 40 F.The air and granular solid material in tube l5 move concurrently upwardat a superficial gas velocity of 2-20 feet per second where the particlesize is from, say, 20-50 mesh. Due to the combined chilling effect ofthe cold solid and the coolant in space 20 the water in the air isfrozen, but the motion of the particles prevents any accumulation offrost on them or on the surface of the tubes.

There may be some slippage of air past the solid inert particles. but inorder that the particles have suflicient scouring action to preventdeposition of ice on the tube surface, superficial air velocity shouldnot be much above that required to suspend the inert solid in the airstream. The ice is carried through tube It by the air as a fine dustmixed with the inert granular solid. From the top of tube IS the mixtureof cold dry air, ice dust. and inert granular solid passes into adisengaging space 40. Here, due to the lowering of the superficial gasvelocity to the order of say 0.2 to 3 feet per second, the inertgranular solid separates from the air and gravitates through tubes 22,where it is rechilled, into the lower chamber 24 of the dryer l2whenceit is again sucked into the lower end of tube i5 by the incomingair. The ice dust, which is finely divided and has a much lower specificgravity than the inert solid, is retained in suspension in thedisengaging space and is withdrawn with the cold dry air from the top ofthe dryer I2 through From the dryer thecold dry air amazes withsuspended ice dust passes into the centrifugal separator mounted on thetop of the tower 50 superposed on and in open communication with heatexchanger 3. The ice dust removed from the air by the separatorgravitates through the tower 50 and contacts the tubes 4 of heatexchanger 3, whereupon the ice is melted and withdrawn through acondensate outlet pipe 8.

The cold substantially dry air freed of ice is then withdrawn from tower50 through pipe 55 and passed into heat exchanger 5 in the spacesurrounding the tubes I therein, where it abstracts heat from theincoming wet air passing through the tubes 1 and finally the dried airis recovered from the system through line 60. Water condensed out of theincoming air in exchanger 5 is withdrawn beyond the header 6| throughpipe 62.

In Fig. 2, I have shown a modification of my invention in which a bankof tubes 26, shown in cross section, containing the coolant are disposedhorizontally within the dryer and circulation of the solid inertmaterial is indicated by the arrows in this figure. In this alternatemethod of operation, the circulating inert solid passes around tubescontaining coolant, rather than through tubes surrounded by coolant. Inthe drawings, I have shown merely one riser tube I5 in the dryer, but itis to be understood that two or more such tubes may be used. Also, boththe riser tube i5 and the coolant tubes 20 may be provided with spiralbafiies or fins in order to cause a turbulent fiow of the fluids throughthe same, and any other known expedient may be used which will increasethe efilciency of the process.

In summary, the method of operation of.the dryer is as follows: Theincoming air is very rapidly chilled by contact with the granular solidas the air passes through the heat exchanger, as indicated previously,and the moisture is removed from the air as ice. However, the ebullientand turbulent state of the solid granules prevents the deposition offrost or ice on the tube wall, and, to a large extent, on the surface ofthe particles of inert solid, and also prevents agglomeration of the iceparticles themselves. The inert particles are carried through thecentral tube and into the disengaging space, from which theygravitateback down through the heat exchanger where they are rechilledand thence discharged to the bottom of thedryer and the cycle isrepeated. The ice dust, because of its small particle size andcomparatively low specific gravity, is not separated in the disengagingspace but is entrained in the cold dry air passing out of the dryer,from which it is subsequently separated, as explained. It is animportant feature of my invention that the velocity of the air and theparticle size of the inert material are so adjusted that the ice dust iscarried overhead with the air, but the inert solid is not withdrawnoverhead through outlet pipe 45.

It should be pointed out that any inert material may be used provided itis dense and hard and does not dust readily. Such solids include, forexample, sand, quartz, sintered clays, hard and rust-resistant metalshot, and the like.

Assume air at 70 F. and containing 1.1 lb. of water per 1000 cubic feetis to be dried. We may assume that the solid material in pipe ll issand, since this material is perhaps as good as any from the standpointof non-dusting, etc. In a representative run, 0.5 lb. of ice are meltedin the'first heat exchanger 3 per 1000 cubic feet of air, during theprechilling of the entering air. Conditions are also controlled so thatthe incoming air in pipe I is at a temperature of about 40 F. The sandor other contact material which is at a temperature of about 20 F. to 30F. in pipe l5, preferably is of a size of from about 20 to 50 mesh. Thecoolant in space 20 which is preferably brine solution (although othercoolants such as liquefied sulfur dioxide, ammonia, etc.,'may be used)is preferably at a temperature of about 40 F. The air velocity in tubel5 may vary from 2 to 20 feet per second (superficial velocity) and from0.2 to 3 feet per second in the disengaging space. The air leaving dryeri2 is at a temperature of about 10 F. It is possible to reduce the wetair mentioned above (1.1 lbs. water per 1000 cubic feet air) to a watercontent of about 0.2 lbs. per 1000 cubic feet of air.

Numerous modifications of my invention may be made by those who areskilled in the art without departing from the spirit thereof.

What I claim is:

1. A method of drying gas which comprises passing a moisture-containinggas upwardly through a cooling zone containing a body of granularmaterial at a velocity sufiicient to keep said granular material insuspension within said .gas, maintaining said cooling zone below thetemperature of said incoming gas and sufiicient to freez the moisturecontained in said gas, removing gas containing ice particles from theupper end of said cooling zone and separating the ice particles fromsaid gas.

2. A method of drying gas which comprises passing a moisture-containinggas upwardly through a cooling zone containing a body of granularmaterial at a velocity sufficient to keep the granular material insuspension, maintaining said cooling zone below the temperature of theincoming gas stream and sufilcient to freeze moisture contained in saidgas, removing a suspension of granular material, cooled gas and iceparticles from said cooling zone, separating the granular material fromsaid gas and ice particles, cooling said granular material below thetemperature maintained in said cooling zone, returning the cooledgranular material to said cooling zone and removing ice particles fromsaid cooled gas.

3. In the process defined by claim 2, the further improvement whichcomprises reducing the velocity of the gas after removal from saidcooling zone to cause said granular material to separate therefrom.

WALTER F. ROLLMAN.

REFERENCES CITED The following referenioes are of record in tho file ofthis patent:

UNITED STATES PATENTS Number Name Date 83,255 Chicester Oct, 20, 18681,148,331 Olsson July 27, 1915 1,974,145 Atwell Sept. 18, 1934 economy,availability.

